The present invention relates to a method of calibrating a measured optical density and apparatus for practicing this method by the use of standard optical density plates.
Two types of optical densitometers are well known: transmitted-light densitometers for detecting transmission densities of measured subjects, and reflected-light densitometers for detecting reflection densities of measured subjects. These densitometers are widely used in image recording such as photography, printing and so on to measure characteristics of image recording materials or images recorded on image recording materials. In scientific measurement fields, a film for measuring pressure is used together with a reflected-light densitometer. This pressure-measuring film changes color or magenta density with pressure change. The film is placed where the pressure is applied so to be changed in color. Then, the colored film is measured to detect reflection density. With reference to a conversion table of density to pressure, the pressure is obtained from the measured optical density.
Conventional optical densitometers comprise a body incorporating various elements such as operation circuits, meters, a power source and the like; and a separate measuring head connected to the body for detecting reflected or transmitted light from a subject to be measured. Such a separate measuring head is provided with a light projecting means for projecting light toward the subject, a light receiving means for receiving light from the subject, and a reference light detecting means for detecting a part of the projected light as a reference light.
Generally, when using a densitometer, it is necessary to perform a zero point correction of the densitometer prior to an actual measurement of a subject for the purpose of avoiding errors caused by extraneous factors such as temperature and conditions of ambient light, and by internal factors such as changes of lamp and light receiving elements due to aging. For the zero point correction, members having known densities are measured to compensate for a difference between a measured density and an actual density.
In practice, a low standard density member is measured by the densitometer to be corrected. According to the measured value, which is referred to as a standard value of low density, the needle of a meter of the densitometer is adjusted to register the standard value of low density. Thereafter, a high standard density member is measured by the densitometer. In the same manner, the meter needle is adjusted to register the standard value of high density. The adjustment is repeated several times so as to adjust the meter needle to register the standard values of known low and high densities for zero point correction.
In the conventional zero point correction methods, several needle adjustments are needed to make a zero point correction which is quite a troublesome operation.
The conventional densitometers, because of their separate body and measuring head, are inconvenient to handle and transport. Such a measuring head has a measuring surface attached to a light shielding plate formed with a measuring aperture which is positioned on a subject to be measured. Therefore, if the subject is small, it is troublesome to accurately position the measuring aperture of the subject.